InP quantum dots: Electronic structure, surface effects, and the redshifted emission

We present pseudopotential plane-wave electronic-structure calculations on InP quantum dots in an effort to understand quantum confinement and surface effects and to identify the origin of the long-lived and redshifted luminescence. We find that ~i! unlike the case in small GaAs dots, the lowest unoccupied state of InP dots is the G1c-derived direct state rather than the X1c-derived indirect state and ~ii! unlike the prediction of k•p models, the highest occupied state in InP dots has a 1sd-type envelope function rather than a ~dipoleforbidden! 1p f envelope function. Thus explanations ~i! and ~ii! to the long-lived redshifted emission in terms of an orbitally forbidden character can be excluded. Furthermore, ~iii! fully passivated InP dots have no surface states in the gap. However, ~iv! removal of the anion-site passivation leads to a P dangling bond ~DB! state just above the valence band, which will act as a trap for photogenerated holes. Similarly, ~v! removal of the cation-site passivation leads to an In dangling-bond state below the conduction band. While the energy of the In DB state depends only weakly on quantum size, its radiative lifetime increases with quantum size. The calculated ;300-meV redshift and the ;18 times longer radiative lifetime relative to the dot-interior transition for the 26-Å dot with an In DB are in good agreement with the observations of full-luminescence experiments for unetched InP dots. Yet, ~vi! this type of redshift due to surface defect is inconsistent with that measured in selective excitation for HF-etched InP dots. ~vii! The latter type of ~‘‘resonant’’! redshift is compatible with the calculated screened singlet-triplet splitting in InP dots, suggesting that the slow emitting state seen in selective excitation could be a triplet state. @S0163-1829~97!04428-7#